Data storage systems generally store data on physical media in a manner that is transparent to host computers. From the perspective of a host computer, data is stored at logical addresses located on file systems, or logical volumes, of the storage system. The file systems or logical volumes are typically configured to store the data required for a specific data processing application. To function, data storage systems map the logical addresses to addressable physical locations on storage media, such as direct access hard disks. In a typical configuration, physical locations comprise tracks on a hard disk, and a track can typically store many blocks of data.
Continuous Data Protection (CDP) is a term used to indicate techniques for restoring data sets, such as logical volumes, to a situation existing at a prior instant of time. When a system utilizes CDP, a system administrator does not have to initiate the process of copying logical volumes so as to be able to recover the status at a prior time. Rather, the system creates copies automatically, and the administrator may access the copies by specifying the desired prior time for data restoration.
U.S. Patent Application Publications 2005/0076262, 2005/0076261, 2005/0066222, 2005/0065962, and 2005/0063374 to Rowan, et al., whose disclosures are incorporated herein by reference, describe “a method, apparatus, and system for accessing units of storage that depends at least in part on an address of the unit of storage and the time that data was written to the unit of storage.” A data store is provided that identifies the location of data at a requested address in response to a time specification and a data request. In one embodiment, “the location is chosen from a time store associated with a data store and a current store associated with the data store, or some combination. The identifying step can include determining whether the data at the requested address at the specified time was changed after the specified time. If data at the requested address was changed after the specified time, the identified location is the time store. If the data was not changed, the identified location is the current store.”